1. Field of the Invention
The present invention relates to a laser irradiation apparatus in which a pulsed laser beam emitted from a first laser light source and a pulsed laser beam emitted from a second laser light source are guided to pass through the same optical path for irradiation of an object to be irradiated with the laser beams.
2. Description of the Related Art
Conventionally, a laser irradiation apparatus in which two laser light sources (laser resonators) each which emit a pulsed laser beam with a predetermined frequency are provided and a desired range of an object to be irradiated with a laser beam (e.g., a semiconductor substrate) is irradiated with a pulsed laser beam by using the two laser light sources has been developed (e.g., Patent Document 1 (Japanese Published Patent Application No. 2007-110064)). FIG. 9 shows a structural example of such a laser irradiation apparatus. As shown in FIG. 9, the laser irradiation apparatus is provided with a first laser resonator 31, a second laser resonator 32, a pulse control device 33, an optical path combining optical member 35, a beam expander 37, a cylindrical lens array 39, and a condenser lens 41.
The first laser resonator 31 emits a linearly-polarized pulse laser beam, of which the polarization direction is perpendicular on the plane of the paper of FIG. 9, with a predetermined frequency. The second laser resonator 32 emits a linearly-polarized pulse laser beam, of which the polarization direction is in an up and down direction on the plane of the paper of FIG. 9, with a predetermined frequency.
The pulse control device 33 controls the first laser resonator 31 and the second laser resonator 32 so as not to synchronize timing of emission of pulsed laser beams from the first laser resonator 31 and the second laser resonator 32.
The optical path combining optical member 35 can guide the pulsed laser beams to pass through the same optical path using the fact that the polarization directions of the pulsed laser beams from the first laser resonator 31 and the second laser resonator 32 are at 90° to each other. The optical path combining optical member 35 is a polarization beam splitter, for example, which reflects a pulsed laser beam polarized linearly in a perpendicular direction on the plane of the paper of FIG. 9 and transmits a pulsed laser beam polarized linearly in an up and down direction on the plane of the paper of FIG. 9. In this manner, with the use of the optical path combining optical member 35, the pulsed laser beams from the first laser resonator 31 and the second laser resonator 32 are guided to pass through the same optical path; accordingly, the frequency of a pulsed laser beam can be doubled and the power of a pulsed laser beam can be increased.
The beam expander 37 adjusts each of pulsed laser beams from the optical path combining optical member 35 so that the shapes thereof have an elongated shape. Each of the pulsed laser beams which have passed through the beam expander 37 is adjusted so that they have a cross-section with an elongated shape (e.g., a linear shape or a rectangular shape) in a direction perpendicular to the traveling direction of the pulsed laser beams on the surface to be irradiated with the laser beam of the object to be irradiated with the laser beam (e.g., a semiconductor substrate). In FIG. 9, the cross-sectional shapes are adjusted to have an elongated shape in an up and down direction in FIG. 9.
The cylindrical lens array 39 divides an incident pulsed laser beam into plural beams. The condenser lens 41 superimposes these divided beams on the surface to be irradiated with the laser beam of the object to be irradiated with the laser beam. Note that the reference numeral 43 denotes a short-side direction condenser lens which concentrates the pulsed laser beam on the surface to be irradiated with the laser beam with respect to a perpendicular direction on the plane of the paper of FIG. 9.
While the surface of the semiconductor substrate is irradiated successively with a pulsed laser beam with the above-described laser irradiation apparatus, the semiconductor substrate is transferred in a perpendicular direction on the plane of the paper of FIG. 9. In this manner, a desired range of the surface of the semiconductor substrate can be irradiated with the pulsed laser beam. Note that as an example of a prior art reference other than Patent Document 1, Patent Document 2 (Japanese Published Patent Application No. 2004-95792) can be given.
In the case where a semiconductor substrate is irradiated with a laser beam with the use of the laser irradiation apparatus in FIG. 9 for laser annealing treatment of the semiconductor substrate, there is a possibility that an object to be irradiated with a laser beam may be adversely affected by the difference in the polarization state between pulsed laser beams.
An average size of crystal grains in a crystallized semiconductor is different between the case where laser annealing is performed by irradiating a desired range of a surface to be irradiated with a laser beam of a substrate on a surface of which an amorphous semiconductor is provided (hereinafter also referred to as an “amorphous semiconductor substrate”) with only an s-polarized pulse laser beam and the case where laser annealing is performed by irradiating a desired range of a surface to be irradiated with a laser beam of an amorphous semiconductor substrate with only a p-polarized pulse laser beam. Here, the term “s-polarized” means a polarization state in which the direction of the electric field of a beam intersects with the traveling direction of a laser beam and is parallel to an up and down direction of the plane of the paper of FIG. 9 on a surface to be irradiated with a laser beam. The term “p-polarized” means a polarization state in which the direction of the electric field of a beam intersects with the traveling direction of the laser beam and is parallel to a perpendicular direction on the plane of the paper of FIG. 9 on a surface to be irradiated with a laser beam. FIG. 10 is a graph showing such a difference. In FIG. 10, the horizontal axis represents the energy density of a pulse laser beam with which a surface of an amorphous semiconductor substrate is irradiated, and the vertical axis represents the average size of crystal grains in a semiconductor crystallized by laser annealing. The squares represent measurement results in the case where the amorphous semiconductor substrate is irradiated with only an s-polarized pulse laser beam, and the diamonds represent measurement results in the case where the amorphous semiconductor substrate is irradiated with only a p-polarized pulse laser beam. As shown in FIG. 10, the size of crystal grains in a semiconductor, which are grown with an s-polarized pulse laser beam, is different from the size of crystal grains in a semiconductor, which are grown by a p-polarized pulse laser beam. Thus, when an amorphous semiconductor substrate is alternately irradiated with an s-polarized pulse laser beam and a p-polarized pulse laser beam, regions which are irradiated with only an s-polarized pulse laser beam and regions which are irradiated with only a p-polarized pulse laser beam are generated in some cases. As a result, there is a possibility that the size of crystal grains may be nonuniform; accordingly, a stable crystalline semiconductor cannot be obtained. As described above, there is a possibility that an object to be irradiated with a laser beam may be adversely affected by the difference in the polarization state between pulsed laser beams.